HOBBS — MINERALOGY OF WISCONSIN. 125 



the component individuals at the ends of the principal 

 axes a diameter of about one to two centimeters. 



On specimens from Highland are found interesting arbo- 

 rescent forms not unlike the forms assumed by native cop- 

 per (see figure 6). The individual cubes are much elongated 

 in the direction of a principal axis and otherwise distorted, 

 and the arborescent groups of crystals are often attached 

 by the end of a single crystal, so that their resemblance to 

 trees is very striking. 



Fig. 6.— Arborescent Form op Galena prom Highland. 



Polysynthetic tv/in lamellae like those described by 

 Cross ' have been observed on crystals from several of these 

 localities. On crystals from Highland the twinning plane 

 of the lamellae is the octahedron, which corresponds to the 

 second law mentioned by Cross. The crystals on which 

 the lamellae are observed, are cubes with small trunca- 

 tion by the octahedron. The faces of the cube are divided 

 along lines parallel to their own edges into four sectors, 

 in each of which pronounced striations run parallel to the 

 adjacent octahedral face. All the cubic faces are affected 

 in the same manner, but the octahedral faces are quite 

 plane. This structure is indicated in plate 6, fig. 7. 



Lamellar twinning according to the other law deter- 

 mined by Cross, where the twinning plane is a plane of 

 the dodecahedron, is well shown by crystals from Mineral 

 Point. The appearance of these crystals is shown in plate 



» Whitman Cross, Note on Slipping Planes and Lamellar Twinning in Galena. Proc 

 CoL Sci. Soc, 2. p. 171-174. (1887). 



